Turbochargers are frequently utilized to increase performance of an internal combustion engine. A turbocharger can extract energy from an engine's exhaust via a turbine to drive a compressor that compresses intake air directed to the engine. Turbochargers typically rely on a radial or centrifugal compressor wheel or wheels. A single compressor wheel may have a single face or two faces (e.g., arranged back to back). In general, intake air is received at an inducer portion of a face and discharged radially at an exducer portion. The discharged air is then directed to a volute, usually via a diffuser section.
A compressor may be characterized by a compressor flow map. A compressor flow map (e.g., a plot of pressure ratio versus mass air flow) can help characterize performance of a compressor. In a flow map, pressure ratio is typically defined as the air pressure at the compressor outlet divided by the air pressure at the compressor inlet. Mass air flow may be converted to a volumetric air flow through knowledge of air density or air pressure and air temperature.
Various operational characteristics define a compressor flow map. One operational characteristic of a compressor is commonly referred to as a surge limit, while another operational characteristic is commonly referred to as a choke area. A map may be considered as presenting an operating envelope between a choke area or line and a surge area or line.
Choke area results from limitations associated with the flow capacity of the compressor stage. In general, compressor efficiency falls rapidly as the local Mach number in the gas passage approaches unity. Thus, a choke area limit typically approximates a maximum mass air flow.
A surge limit represents a minimum mass air flow that can be maintained at a given compressor wheel rotational speed. Compressor operation is typically unstable in this area. Strong fluctuation in pressure and flow reversal can occur in this area, hence continuous operation is not desirable.
In general, compressor surge stems from flow instabilities that may be initiated by aerodynamic stall or flow separation in one or more of compressor components (e.g., as a result of exceeding a limiting flow incidence angle to compressor blades or exceeding a limiting flow passage loading).
For a turbocharged engine, compressor surge may occur when the engine is operating at high load or torque and low engine speed, or when the engine is operating at a low engine speed with a high rate of exhaust gas recirculation (e.g., EGR). Compressor surge may also occur when a relatively high specific torque output is required of an engine with a variable nozzle turbine (VNT) or an electrically assisted turbocharger. Additionally, surge may occur when a rapid intake air boost is initiated using an electric motor or VNT mechanism, or when an engine is suddenly decelerated (e.g., consider a closed throttle valve while shifting gears).
Various technologies described herein pertain to compressor assemblies where, for example, one or more components can widen a compressor map by delaying surge.